Acoustic tympanum



Sept 9, 1930. J. A. PoRoMA 1,775,294

ACOUS TIC TYMPANUM Filed Feb. l2., 1929 wvefntoz Patented Sept. 9,' 1930 .UNITEDl STATES JOHN A. POROMA, F NEW YORK, N. Y.

ACOUSTIC TYMPANUM Application filed February 12, 1929. Serial No. 339,289.

This invention relates to improvements in diaphragms, particularly to those employed in the reproduction of sound, and more espe-- cially to the class thereof designed for reproducing sound directl in free air without the use of a horn or li e medium; and the object of the invention is to provide a tympan by means of which sound vibrations will be faithfully reproduced and that will have no lo audible resonance timbre or vibration period of its own.

-I have found that in diaphragms of the type whereto this invention appertains, distort-ion of sound is caused by high or uncontrolled resilience, .whether inherent in the material from which the diaphragm is made, or, through stresses and tension produced by the construction thereof, subjecting the diaphragm to sustained or prolonged sound vibratons. The origin of these vibrations commences from a point at which the actuating unit or motor is connected to the diaphragm, and extend thence outwardly through the latter in a series of concentric waves or iniiections; each succeeding wave having an, increased radius and a subsequently lower pitch, and in consequence thereof adding to each period of vibration imposed on the diaphragm by the actuator, a series of sound inflections created by the recoil action of the diaphragm,`such inflections producing an echo-effect known as lag of vibrations'which reduces the audibility of overn tones, besides making the reproduction indistinct. I

In order to obviate the hereinbefore mentioned defects, I `have found it necessary to reduce the springiness of the diaphragm, by using material of low resiliency and elimi- 74719, 'nating stresses and tension in the construction thereof, as well as to provide the diaphragm with radially disposed, oppositely arranged corrugations having unequal resilience and contrasting vibration periods.

The invention comprises anon-stressed diaphragm, having formed therewith a plurality of radial corrugations including alternatel disposed ridges and valleys arranged side y side and diverging from a common centre outwardly towards the margin of thev diaphragm. The facial area of each corrugation, is bounded laterally by a pair of ridges or two valleys and may be triangular, sectoral or trapezoidal in form. The dihedral and reflex angles between the faces of adjacent corrugations may Vary from a minimum, or almost zero, near the centre of the diaphragm, where these angles are acute, te

a maximum obtuse angle at the diaphragms perimetral edges. The variations between the aforesaid angles depend upon the depth or height of the corrugations at the outer limits thereof. The before mentioned dihedral angles on the opposite inner and outer faces of the diaphragm are re-enforced to give them more or higher resiliency that is, unequal to the low elasticity of the sides or side-fields of the corrugations.

. One embodiment of this improved diaphragm is represented on the annexed drawing, wherein Fig. I is a front or face view thereof, parts of the same being broken away; y

Fig. II is a cross-section on the line II-II of Fig. I, looking upwardly thereon accordin to the arrows;

ig. III is a complete front rView of the diaphragm alone, shown on a reduced scale,

and illustrating also the comparative size thereof, before the corrugations are incorporated therein;

Fig. IV is an' enlarged cross-section taken on the line IV-IV of Fig. I, looking in the direction of the thereto appertaining arrows;

Fig. V is a similar cross-section taken on, the line V-V of the same figure; and

Fig. VI is a section taken transversely across one of the corrugations of a modified type of diaphragm.

.In these views, Fig. III shows one of several diiferent ways of making this improved diaphragm. The method followed, consists in taking a sheet of. paper of medium thickness, denoted by the reference numeral 11, 95 of an area, larger than the quadrantal dimension that the diaphragm will finally have, and providing the latter centrally with an opening 12. Next, .sheet 11, represented by dot-and-dash lines, in Fig. III is folded,

' pressed lor otherwise formed with corrugations 13 having relatively sharp cornered ridges and valleys, 14 and 15, respectively. The act of incorporating these corrugation's inAsaid sheet 11, shrinks the same all the way around, so as to draw its perimeter together to the size thereof represented by thc full line bounding edges 16, in Figs. I and III. They ridges and valleys spoken of, in the forming thereof, contract opening 12, closing the latter and causing the apices of the radiallytapering corrugations to approach or come together adjacent one another in proximity of their common centre in the middle of the diaphragm.

It will be observed with particular reference to Fig. I that in cach quarter section of the substantially square diaphragm, there appears four complete corrugations, or as -many ridges and valleys.'` From Fig. II' it will be seen that the ridges 14 of the corrugations are disposed in one plane from the perimeter towards the centre of the diaphragm, while the valleys 15 slope downwardly outward therefrom. The height or depth at the base or outer end of a corrugation, predetermines the dihedral angle ineluded between the faces of adjoining sides of a pair of corrugations. In the device depicted on the accompanying'drawing, the

-diEerence of the dihedral angles, intermediate the valleys smaller and Ilarger ends, is abouty 45 degrees. But this can vary and may even-be uniform from end to end of the valleys depending upon the depth of the corrugations, as before pointed out.

By reason of the fact thatthe centre of the diaphragm liesapproximately flush or in the same plane as the tops 0f the ridges 14, there is afforded beneath the centre of .the diaphragm a space wherein theactuating unit or vibration imparting motor may be conveniently housed. Neither this motor nor the supports therefor have been shown, as

these details can be varied according to cir-y cumstances. Similarly, the number of corrugations, depths and lengths thereof, as well as the perimetral contour of the diaphragm may also be changed to suit conditions or sizes desired.

To the opposite sides of the diaphragms central portion is secured, as by l g'lueing,

washers 17 of paper or other fibrous material. `Through these washers is inserted the outer threaded extremity of a spindle A, pertaining to the actuating motor. Nuts 18y engaging the spindle and bearing against the outer and inner washers, serve to anchor said spindle to the'diaphragm for. transmitting the vibrations thereto. y Y

In the gutters ofthe valleys, on one or both sides of the diaphragmythe same is provided with a coating or lining 20 of some glutenous or viscid substance, such as shellac, latex or the like, which serves asa re-enforcement for these converse and inverse angles, to render them stiffer than the field elements 21 intervening between the high and low lines of demarcation of the ridges and valleys. The re-enforcing substance hereinabove referred to, which is visible on the bottoms or `gutters of the valleys, have been indicated in Fig. I, by angularly disposed short and full lilies, while the same material on the reverse-valleys or the under-sides of the ridges, are depicted by similar but dotted lines. The re-enforcements-in question, in giving additional rigidity to the tops and bottoms of the before named corrugations, afford them a contrasting vibration period to that of the sides or fiat fields 21.therebetween, but at no time is any appreciable stress applied to the diaphragm in the process of manufacture. It will be understood that the radial sections of the diaphragm are thus equal mass, or weight and springiness. ,'llhe inclined eld elements 21, a pair of which compose each corrugation, are substantially triangular or trapezoidal for a quadrantal diaphragm, and sectoral vfor one that is of circular or disc formation. The very apices of these angularly converging field elements are removed or squeezed together to receive the washers 17 to which the actuator rod A is anchored.

The perimetral edges all the way around the diaphragm are suspended or connected to a surrounding frame 25, of wood or other appropriate material, by means of a fieXible member 26, for which a band or fabric tape may be employed.'v This band is cemented to the diaphragm a certain -distance inwardly `from its edges, following the outer limit-s of the elds thereof a specified amount, up and down over the ridges and valleys.v The marginof band 26 is preferably secured to one of the narrow and as shown, the outer face of said frame 25, through the agency of moldings 27, tacked to the latter over the said fabric member. It is obvious that tapestry, a-painting, or4 any other ornamental covering medium may be stretched over the outwardly turned faee'of the frame to shield the diaphragm and afford the ensemble a finished appearance without detracting from the transmission of sound.

In the modified form of the invention depicted in Fig. VI the re-enforcements to the ridges and valleys, consist of lips or flanges 20a,A formed with the sides or field elements of the corrugations. These flanges are glued or cemented to the underside of adjoining element-s, so as to be hidden thereby. -It is even conceivable that the diaphragm with its re-enforced ridges and valleys could be moulded or pressed in one piece of a composition suitable therefor.

In operation, the diaphragm receiving a vibration or'impulse from the operating unit,

`is bodily carried ina direction parallel with that of the movement ofthe actuating spindle A. The inclined field elements composing thesurface of the diaphragm are disposed at acute angles to this direction of motion, and will, in consequence of the resistance thereto, by virtue of the rigidity of said fields, set up airwaves harmonious with each such motion or stroke from the actuating motor. A series of these a1r waves as well known, are concelved as sound. The

sides or field elements of the corrugations are substantially in a nontensioned state, individually as well as collectivelyy as a diaphragm, being thus inert and incapable of recoiling or rebounding to create or prolong any vibrations of themselves. The re-en. forced angles at the ridges and valleys preclude the setting up of concentric reverberations in the diaphragm. The higher vibration periods of these stienedv angles are dampened or tuned down by the low vibration period of the fields lying between the said angles.

Having described my invention, what I desire to secure by Letters Patent and claim 1. The method of producing an acoustic diaphragm which consists in providing a sheet of appropriate material with a central opening, and contracting the latter by incorporating in the remainder of said sheet radially diverging corrugations.

2. The method of producing an acoustic diaphragm which consists in providing a sheet of suitable material with radially diverging alternately disposed ridges and valleys, and applying a re-enforcing medium to the bottoms ofthe valleys at one side of the diaphragm.

' 3. An acoustic diaphragm formed of a i set towards one side thereof.

6. An acoustic diaphragm /havingthe entire area thereof provided with radially diverging substantiall unstressed flat fields, accentuated lines o the latter in a lengthwise direction, said fields demarcation dividing` mum adjacent thecentre of the diaphragml to an obtuse angle at its perimetral ed es.

7 An acoustic diaphragm having ra ially diverging alternately disposed ridges and valleys, formed throughout its facial area, oppositely inclined unstressed flat fields intervening between said ridges and said valleys, and means re-enforcing thevbottoms of the latter to vary the vibration period there-l of from that of the said fields.

8. An acoustic diaphragm having its vsuperfcial area formed with'radially diverging alternatel disposed oppositely inclined ridges an valleys, and a re-enforcing medium applied to the bottoms of said valleys at one side of the-diaphragm.

9. An acoustic diaphragm formed with radial sections having different portions of unequal mass and springiness.

' JOHN A. POROMA.

and said demarcation lines forminginclined ridges and valleys, the `dihedral angles between the said elds'varying from a mini- 

